ARABIDOPSIS-THALIANA DNA METHYLATION MUTANTS

Three DNA hypomethylation mutants of the flowering plant Arabidopsis t haliana were isolated by screening mutagenized populations for plants containing centromeric repetitive DNA arrays susceptible to digestion by a restriction endonuclease that was sensitive to methylated cytosin es. The mutations are recessive, and at least two are alleles of a sin gle locus, designated DDM1 (for decrease in DNA methylation). Amounts of 5-methylcytosine were reduced over 70 percent in ddm1 mutants. Desp ite this reduction in DNA methylation levels, ddm1 mutants developed n ormally and exhibited no striking morphological phenotypes. However, t he ddm1 mutations are associated with a segregation distortion phenoty pe. The ddm1 mutations were used to demonstrate that de novo DNA methy lation in vivo is slow.